High-energy solid compositions, such as solid propellants, explosives, gasifiers or the like, comprise an elastomeric binder in which is dispersed particulate solids, such as particulate fuel material and/or particulate oxidizers. High-energy compositions typically include a liquid plasticizer, such as a nitrate ester plasticizer, which contributes to the elastomeric characteristics of the binder and adds energy to the composition.
Of particular interest herein are cross-linked elastomers formed from a hydroxyl-terminated polyester or polyether prepolymer plus a curative. Examples of relatively non-energetic polyester and polyether prepolymers are polyethylene glycol (PEG), polycaprolactone (PCP), and polydiethylene glycol adipate (PGA). An example of an energetic prepolymer is Glycidyl Azide Polymer (GAP).
Polyethers useful as prepolymers for binders may be formed by polymerizing oxetanes having pendant groups, as described in U.S. Pat. No. 4,483,978, issued to Manser. Energetic pendant groups, e.g., cyano, azido, and nitrato, contribute to the energy of the prepolymer and the binder and compositions formed therefrom.
Hydroxyl-terminated polyethers and polyesters useful as prepolymers in high-energy compositions typically have functionalities of about 2, i.e., the functionality provided by the terminal hydroxyl groups. For an elastomer to function as a binder in a high-energy composition, substantial networking must be established with a curative, and a curative having a functionality substantially higher than 2 is required, preferably in the range of about 3.
The standard polyfunctional polyisocyanate used today in high-energy compositions, such as propellants, is Mobay's Desmodur N-100.TM. (hereinafter N-100). N-100 is synthesized by the controlled reaction of hexamethylene diisocyanate with water. The result is a mixed product, and several studies have indicated that there are at least four principal products in N-100. The functionality of N-100 is approximately 3.5.
Although N-100 has proven to be a useful and effective curative for propellant compositions, there are problems with N-100 which might be addressed by a different curative. The 3.5 functionality of N-100 is higher than the more ideal functionality of 3.0 for hydroxyl-terminated prepolymers, resulting in excessive cross-linking in certain cases. Biuret groups formed in the synthesis of N-100 are stiff because of hydrogen bonding, and this is believed to reduce propellant mechanical properties. N-100 has a high equivalent weight (molecular weight per functional group), i.e., about 197; as N-100 is non-energetic, weight is added without adding energy. Being that N-100 is a mixed product, there is undesirably high variability from lot to lot.
It has been proposed to increase propellant performance by switching to a lower molecular weight, purely trifunctional isocyanate. 1,3,5-triisocyanatopentane (PTI) has been proposed for this purpose. Unfortunately, while some binders appeared promising, PTI proved unable to cross-link certain prepolymers, particularly GAP, which would not react with the secondary isocyanate group in PTI.
The need exists for improved isocyanate cross-linking agents for use in high-energy binders, particularly for solid propellant binders.